increasing noise levels in bars as the night wears on

It would be interesting to chart the noise levels at a bar where there's background music playing all night long, versus a bar where there's no music playing. Some places are just so much louder than others, even though there's really no reason why they would be. Maybe the people are drunker? Maybe the space is more echoey? I wonder if the background noise is always rising... people talking louder to be heard over the noise, increasing the noise, forcing the next round to speak louder still. I also wonder whether a positive noise feedback loop gets triggered at a certain noise level, below which the noise stays relatively level but above causing things to get louder and louder over time. Or if one particularly loud song is enough to raise the ambient noise to a point that's self-sustaining even after the song ends or is turned off. I wish there were some way to answer this question easily, as my curiosity is about strong enough to try a couple Google searches but not strong enough to do a study like this.